The object of this invention is a material based on titanium oxysulphide, which can be used, in particular as a positive electrode in an electrochemical generator or an electrochromatic cell.
In a more precise way, it relates to the creation of electrochemical generators or electrochromatic cells in particular in thin layers, whose principle of operation is based on the insertion and the removal of alkali metal ions or protons into and from the positive electrode.
Electrochemical generators of this type can find numerous applications, for example, in the form of micro-generators having a total thickness of a few xcexcm in systems for providing back-up to the memories of micro-systems during a power failure or as integrated systems in cards with a memory of the bank card type.
These generators can also be used as an electrochromatic system when the positive electrode changes colour during the insertion, which makes them interesting for the display of information or better still when the negative electrode is transparent, for glazing that changes color according to needs.
Electrochemical generators of this type using a chalcide or an oxychalcide of titanium as a positive electrode, are described in particular, in the document WO-A-90/05387. In this document, the positive electrode material is made up of a layer of compound corresponding to the formula:
TiOaXb
in which X represents an atom of sulphur, selenium or tellurium and a and b are such that a is at the most equal to 2, b is at the most equal to 3 and (a+b) is between 2 and 3.
The layer is prepared by magnetron type cathode sputtering, from a titanium disulphide TiS2 target containing 5% of oxygen atoms.
This electrode material has interesting electrochemical properties but its deposition by cathode sputtering, in the form of a thin layer, has the disadvantage of requiring the manufacture of TiS2 targets, from commercial TiS2 powders which always contain a few percent of oxygen. Indeed, because of the lamellar structure of TiS2, the powder compacts rather easily but the sintering is delicate since there is a loss of sulphur whatever temperature is used. Because of this, during the cathode sputtering, the power applied to the titanium sulphide target has to be limited to a value below 2 Watts/cm2 otherwise one risks seeing the target cleaved depending on the thickness.
Furthermore, although the performance of an electrochemical generator using this electrode material in the form of a thin layer is satisfactory, it would be of interest to further improve this performance, in particular the capacity of the generator per unit mass. Hence research has been undertaken to provide electrode materials of the same type under better conditions and to further improve the performance of the electrochemical generator using such electrodes.
The document U.S. Pat. No. 4,508,608 describes a method of preparation of a cathode based on a high porosity chalcide, by cathode sputtering of molecules of a transition metal chalcide onto and into a high porosity current collector substrate. The chalcide can be, in particular, a titanium sulphide such as titanium disulphide or trisulphide.
The energy per unit mass of titanium trisulphide (840 Wh/kg) is very high in relation to that of titanium disulphide (485 Wh/kg) but the use of TiS3 poses certain problems since the insertion of lithium is only weakly reversible in TiS3.
The precise object of this invention is a positive electrode material, based on titanium oxysulphide which gets around these disadvantages and which leads to a notable improvement in the performance of the micro-generator, in addition having the advantage that the electrodes in thin layers are suitable for manufacture on an industrial scale.
According to the invention, the positive electrode material for an electrochemical generator is made up of a thin layer of an amorphous compound of formula:
TiOaS3+b
in which a and b are such that:
0 less than axe2x89xa60.5
0 less than bxe2x89xa60.7
According to the invention, thanks to the choice of an electrode material of the type TiOaS3+b which can be deposited by cathode sputtering, one obtains a layer of isotropic and amorphous material of much lower density than that of massive TiS3, which, because of this, stands up to the insertion and removal of alkali metal ions or protons very well. In effect, its low density corresponds to an expanded amorphous structure and there is no swelling of the material during the insertion of the ions. Hence, the layer does not deteriorate since the transport of the ions occurs without any change in the morphology. This isotropic layer also offers the advantage of allowing good diffusion of lithium in all directions.
According to the invention, this thin layer of titanium oxysulphide corresponding to the formula mentioned above can be prepared by radiofrequency cathode sputtering from a TiS3 target. In the TiS3 compound, the titanium atom is tetravalent as in TiS2 and the sulphur is present in the form of a sulphide and a disulphide group: Ti4+S2xe2x88x92S22xe2x88x92.
For this deposition, the use of a TiS3 target is of particular interest since TiS3 can be obtained easily by chemical reaction of titanium and sulphur in powder form, in stoichiometric quantities, in a sealed tube under vacuum at 500xc2x0 C. Under these conditions, a black coloured TiS3 powder is obtained, the spectrum X of which corresponds to that of TiS3. This powder can be easily compacted, for example, under a pressure of 294 MPa (3 tonnes/cm2) and sintered, for example, at a temperature of 500xc2x0 C., in a sealed tube under vacuum. Hence, one can manufacture targets of much larger diameter, for example from 50 to 75 mm diameter, than is the case for TiS2 targets. Also, this method can be implemented on an industrial scale for the manufacture of electrodes in thin layers, which was not possible with the method of the prior art using a TiS2 target, because of the need to limit the power applied to the TiS2 targets.
An object of the invention is also an electrochemical generator using the positive electrode material mentioned above. This generator comprises a positive electrode, a negative electrode capable of liberating a proton or an alkali metal ion and an ion conducting electrolyte arranged between the two electrodes, and is characterised in that the positive electrode is formed from a thin layer of an amorphous compound of formula:
TiOaS3+b
in which a and b are such that:
0 less than axe2x89xa60.5
0 less than bxe2x89xa60.7
said layer being arranged on a substrate.
The substrates used to support the thin layer of amorphous compound can be very diverse, conductors or insulators, flexible or rigid. Generally, one uses a substrate that conducts electricity or an insulating substrate covered with a layer of an electrically conductive material onto which the thin layer of the compound TiOaS3+b is arranged.
The electrically conductive material can be, in particular, a metal, for example Cr, Pt, Ni, Al etc. monocrystalline silicon or an oxide conductor of electricity like the mixed oxides of indium and tin (ITO).
The insulating support can be a ceramic material, Pyrex, a glass or a plastic material resistant to the cathode sputtering conditions, for example, a flexible plastic material such as a polyester like the polyterephthalate of ethylene glycol or a polyimide.
These flexible substrates allow, in particular, continuous production of large surface area electrodes since they can be passed continuously into the cathode sputtering deposition enclosure.
In the generator described above, the negative electrode can be produced, in particular, in lithium or in a material containing lithium. The material containing lithium can be an alloy of lithium or a lithium compound.
The negative electrode is preferably in the form of a thin layer deposited by traditional methods, for example by evaporation under vacuum or by cathode sputtering. Thin layers obtained by such methods are highly advantageous since the departure of the ions does not create any void at the interface.
The negative electrode can also be produced in an alloy or in a compound including other alkali metals, for example sodium, potassium, caesium or rubidium or in a compound capable of liberating protons like the metal hydrides such as LaNi5H6 and hydroxides such as iridium hydroxide.
When the negative electrode is made of lithium, a glass can be used as a solid electrolyte that conducts lithium ions. This glass must be an electronic insulator in order to prevent auto-discharge of the generator but its ionic conductivity must be the highest possible.
In effect, the electrolyte must play two essential roles: to be an excellent electronic insulator between the two electrodes, and to be a good ionic conductor. Its thickness must be sufficient and the layer must be absolutely free of defects such as holes or fissures which would have the immediate consequence of short circuiting the generator during the deposition of the negative electrode.
Glasses based on boron oxide, lithium oxide and a lithium salt can be used, for example, glasses containing, in various proportions B2O3, Li2O and LirXxe2x80x2 with Xxe2x80x2 representing an anion capable of combining with lithium in the form of a salt and where r represents the valency of the anion Xxe2x80x2.
As examples of anions Xxe2x80x2 that may be used, halide and sulphate anions may be mentioned.
One may also use glass conductors based on sulphides, for example glasses with boron sulphide, lithium sulphide and a lithium salt.
When the negative electrode includes other alkali metals or protons, the electrolyte can be made up of glasses of the same type containing the same alkali metal ions or protons.
The electrolyte can be in the form of a layer produced by evaporation under vacuum or cathode sputtering. Preferably, cathode sputtering is used to provide a continuous layer of small thickness, free of defects.
As an electrolyte, one can also use solid materials made of an ion conducting polymer, for example a polymer of the polyoxyethylene type.
The electrochemical generator of the invention can be produced by traditional methods, by successively depositing onto a substrate covered with an electrically conductive layer forming a current collector, a first layer of the amorphous compound TiOaS3+b, a second layer of solid electrolyte and a third layer forming the negative electrode.
According to the invention, the first layer of compound TiOaS3+b constituting the positive electrode is deposited by cathode sputtering, preferably by radiofrequency cathode sputtering ; this allows one to obtain a thin, compact, continuous and homogeneous layer, having an extremely even profile without any surface porosity.
The thickness of the TiOaS3+b layer can vary over a wide range. Generally a thin layer having a thickness of from 200 nm to 10 xcexcm is preferred.
This structure is particularly advantageous since the electrolyte deposited afterwards can be in the form of a layer of thickness even smaller than was the case for the xe2x80x9call solid-statexe2x80x9d electrochemical generators of the prior art.
Because of this, even when using electrolytes having low ionic conductivity, higher current densities can be obtained than in the electrochemical generators of the prior art by using smaller thickness"" of electrolytes without the risk of short-circuits.
Furthermore, by choosing an ion conducting glass, organic or inorganic, with conductivity by alkali metal ions such as lithium, the only mobile element is lithium, the anions being locked in the electrolyte structure. Hence the ionic transport takes place through a single element and the system of thin layers facilitates this transport and allows an improved performance to be provided.
The oxygen in the isotropic material TiOaS3+b stems from the oxygen present as an impurity in the TiS3 target.
For the production of the electrochemical generator, the second layer of solid electrolyte and the third layer forming the negative electrode can be deposited by the traditional methods that are suitable for the creation of thin layers. In particular, the solid electrolyte can be deposited by cathode sputtering or by evaporation under vacuum and the negative electrode by evaporation under vacuum.
The principle of operation of the generator described above is based on the insertion and the removal of an alkali metal ion or a proton into and from the positive electrode.
The operation of the generator corresponds to the overall reaction:
Ti4++Lixe2x86x92Ti3++Li+
This corresponds to an electromotive force of 2.945 volts.
This scheme is theoretical since, on the one hand the activity of the lithium ion can be different and, on the other hand, other ionic species stemming for example from the sulphur can participate in the electrochemical reaction.
Other characteristics and advantages of the invention will become more apparent on reading the description that follows, making reference to the appended drawings and given, it is understood for the purposes of illustration and being non-limitative.